The present invention relates to the recovery of hydrocarbons from a subterranean reservoir.
Hydrocarbons that are recovered from a subterranean reservoir include oil, gases, gas condensates, shale oil and bitumen. To recover a hydrocarbon, such as oil, from a subterranean formation, a well is typically drilled down to the subterranean oil reservoir and the oil is collected at the well head. The recovery of hydrocarbons that are very heavy or dense, such as for example, the recovery of bitumen from oil sands, are especially difficult as these materials are often thick and viscous at reservoir temperatures, so it is even more difficult to extract them from the subterranean reservoir. For example, bitumen can have a viscosity of greater than 100,000 centipoises, which makes it difficult to flow. Suitable methods for the recovery of these heavier viscous hydrocarbons are desirable to increase the world's supply of energy. Methods for recovering bitumen are particular desirable because there are several trillion barrels of bitumen deposits in the world, of which only about 20% or so are recoverable with currently available technology.
A conventional method of recovering hydrocarbons from a subterranean oil reservoir is by utilizing both a production well and an injection well. In this method, a vertical production well is drilled down to a hydrocarbon reservoir, and a vertical injection well is drilled at a region spaced apart from the production well. A fluid is injected into the hydrocarbon reservoir via the injection well, and the fluid promotes the flow of hydrocarbons through the reservoir formation and towards the production well for collection. However, a problem with this method is that the injected fluids tend to find a relatively short and direct path between the injection and production wells, and therefore, bypass a significant amount of oil in the so called “blind spot”. Furthermore, if the injected fluid, such as steam, is lighter than the reservoir oil, the injected fluid tends to flow through the upper portion of the reservoir and thus bypass a significant amount of oil at the bottom of the reservoir. Due to these unfavorable mechanisms, injected fluids tend to reach the production well at a relatively early time. When this “early breakthrough” of the fluids occurs, the steam-oil ratio increases rapidly and recovery efficiency of the hydrocarbons is reduced.
In one method of improving the recovery of hydrocarbons using vertical injection and production wells, a horizontal high-permeability web is formed at the bottom of the production well to increase the hydrocarbon recovery area at that region, as described in U.S. Pat. No. 6,012,520, which is incorporated herein by reference in its entirety. The high-permeability web has multiple channels or fracture zones that are formed horizontally about a receiving region of the production well located near the bottom of the reservoir. To recover the hydrocarbons, a neighboring injection well injects steam into a top portion of the reservoir via an injection inlet. The injected steam heats the hydrocarbons in the reservoir, and pushes the hydrocarbons downwards for collection by the high-permeability web of the production well.
However, while this method increases the recovery area immediately about the production well and displaces the oil in a “gravity stable” manner, it's extraction efficiency per unit area is low for subterranean reservoirs having viscous hydrocarbons that are difficult to flow under typical injection pressures. Oil recovery from these reservoirs, such as oil sands reservoirs, remains difficult and yet highly desirable.
In one version of a conventional recovery method, a “huff and puff” process is used to recover bitumen from a subterranean oil sands reservoir. In this method, a vertical well bore is drilled to the reservoir and steam is injected towards the bottom of the bore and into the surrounding reservoir. The steam heats the bitumen about the well bore to reduce its viscosity and cause it to flow back to the well bore. When a desired amount of the bitumen has been collected in the bottom of the well bore, the well is pumped off and the oil is collected at the well head. However, the steam typically traverses only the area immediately around the vicinity of well bore which may be only a small portion of the underground reservoir. Thus the amount of oil recovered is limited by the distance the steam can travel before it cools and condenses, and a large portion of the reservoir may not be reached by steam using this method.
In another conventional method, a Steam Assisted Gravity Drainage (SAGD) process is used to recover bitumen from a subterranean reservoir. In this method, a horizontal production well bore is formed near the bottom of the reservoir. A horizontal steam injection well is formed parallel and above the production well bore. The injected steam heats the bitumen between the wells, as well as above the injection well, and gravitational forces drain the heated bitumen fluids down to the production well for collection. However, this method has problems that are similar to those of the huff and puff method. Namely, after the steam from the injection well reaches the top of the reservoir, the bitumen production becomes limited by the extent to which the steam can laterally expand. As heat losses from the steam to the overburden above the reservoir are high, the lateral expansion is restricted, and a large amount of the reservoir may not be reached by the heated steam.
Thus, it is desirable to efficiently recover hydrocarbons from a large are of a subterranean reservoir. It is furthermore desirable to recover dense or viscous hydrocarbons with injection and production wells that provide a heated fluid to the subterranean reservoir.